Energetic limitations have long plagued the development of compact and lightweight untethered power supplies for applications involving powered hand tools, powered yard equipment, or human-scale robotic systems. As an example, the need for an effective portable power supply for human-scale robots has increasingly become a matter of interest in robotics research. Current prototypes of humanoid robots, such as the Honda P3, Honda ASIMO and the Sony QRIO, show significant limitations in the capacity of their power sources in between charges (the operation time of the humanoid-size Honda P3, for instance, is only 20 to 25 minutes). Given the low energy density of state-of-the-art rechargeable batteries, operational times of these systems in the 100 W range are restrictive. Dunn-Rankin, D., Martins, E., and Walther, D., 2005. “Personal Power Systems”. Progress in Energy and Combustion Science, 31, August, pp. 422-465. This limitation becomes a strong motivation for the development and implementation of a more adequate source of power. Moreover, the power density of the actuators coupled to the power source needs to be maximized such that, on a systems level evaluation, the combined power supply and actuation system is both energy and power dense. Put simply, state-of-the-art batteries are too heavy for the amount of energy they store, and electric motors are too heavy for the mechanical power they can deliver, in order to present a viable combined power supply and actuation system that is capable of delivering human-scale mechanical work in a human-scale self contained robot package, for a useful duration of time. Goldfarb, M., Barth, E. J., Gogola, M. A., Wehrmeyer, J. A. 2003. Design and Energetic Characterization of a Liquid-Propellant-Powered Actuator for Self-Powered Robots, IEEE/ASME Transactions on Mechatronics, Vol. 8, no. 2, pp. 254-262.
The total energetic merit of an untethered power supply and actuation system, this system being an untethered robot, portable powered hand tools, or similar systems, is a combined measure of 1) the source energy density of the energetic substance being carried, 2) the efficiency of conversion to controlled mechanical work, 3) the energy converter mass, and 4) the power density of the actuators. With regard to a battery powered electric motor actuated system, the efficiency of conversion from stored electrochemical energy to shaft work after a gear head can be high (˜50%) with very little converter mass (e.g. PWM amplifiers); however, the energy density of batteries is relatively low (about 350 kJ/kg specific work for Li-ion batteries after the gearhead), and the power density of electrical motors is not very high (on the order of 50 W/kg), rendering the overall system heavy in relation to the mechanical work that it can output. One approach to address the problems of low energy density batteries and low power density actuators is to avoid the electromechanical domain and utilize the pneumatic domain.
With regard to a hydrocarbon-pneumatic power supply and actuation approach relative to the battery/motor system, the converter mass is high and the total conversion efficiency is shown to be lower. However, the energy density of hydrocarbon fuels, where the oxidizer is obtained from the environment and is therefore free of its associated mass penalty, is in the neighborhood of 45 MJ/kg, which is about 2 orders of magnitude greater than the energy density of state of the art electrical batteries. This implies that even with poor conversion efficiency (poor but within the same order of magnitude), the available energy to the actuator per unit mass of the energy source is still at least one order of magnitude greater than the battery/motor system. Additionally, pneumatic actuators have approximately an order of magnitude better volumetric power density and a five times better mass specific power density (Kuribayashi, K. 1993. Criteria for the evaluation of new actuators as energy converters, Advanced Robotics, Vol. 7, no. 4, pp. 289-37) than state of the art electrical motors. Therefore, the combined factors of a high energy-density fuel, the efficiency of the device, the compactness and low weight of the device, and the use of the device to drive lightweight pneumatic actuators (lightweight as compared with power comparable electric motors) is projected to provide at least an order of magnitude greater total system energy density (power supply and actuation) than state of the art power supply (batteries) and actuators (electric motors) appropriate for human-scale power output.
With regard to the scale of interest, the main loss mechanisms for mechanical small-scale power generation devices are dominated by surface related effects: primarily viscous friction, coulomb friction, leakage, quenching, and heat loss. Given that all of these mechanisms are surface effects, they become more dominant at smaller scales as the surface area to volume ratio becomes higher. This is the primary reason conventional internal combustion engines have single digit efficiencies below the 1 kW scale. To overcome these loss mechanisms, a power generation device that minimizes as many of these surface effects resulting in higher efficiency is needed.